Sheetlike light-polarizing complex of iodine and a polyvinyl compound with protective surface boric acid-polyvinyl compound complex



SEARCH ROOM July 20, 1943- M. HYMAN, JR.. ETAL SHEETLIKE LIGHT-POLARIZING COMPLEX OF IODINE A POLYVINYL COMPOUND WITH PROTECTIVE SURFACE BORIC ACID-POLYVINYL COMPOUND COMPLEX mea Feb. e, 1945 INVENTOR.

sEARcHhooN UNITED STATES PATENT OFFICE SHEETLIKE LIGHT-POLARIZING COMPLEX OF IODINE AND A POLYVINYL COMPOUND WITH PROTECTIVE SURFACE BORIC ACID-POLYVINYL COMPOUND COMPLEX Mark Hyman, Jr., and Cutler D. West, Cambridge, Mass., assignors to Polaroid Corporation, Cambridge, Mass., a corporation of Dela- Ware Application February 8, 1945, Serial No. 576,850

Claims.

'This invention relates to a new and improved light-polarizing material. This application is a continuation-impart of our co-pending application Serial No. 499,238, filed August 19, 1943, for Light-polarizer and process of manufacture, but now abandoned.

It is one object of the present invention to provide substantially improved light-polarizing sheets or films of the type described in Patent No.

2,237,567, issued April 8, 1941, to Edwin H. Land. suitable polybasic acid derivative. l Another object of the invention is to provide an In carrying out one embodiment of the invenimproved light-polarizing material of the type tion, a sheet of the desired plastic material, such shown in said patent and comprising a sheet or for example as polyvinyl alcohol or a suitable illrn of a molecularly oriented polyvinyl oxy compolyvinyl alcohol derivative or mixed linear polypound, such as polyvinyl alcohol or a suitable l5 mer containing polyvinyl alcohol, may first be cyclic ether of polyvinyl alcohol, having incorpostretched to orient the molecules therein. The rated therewith a dichroic substance such as one vdegree of stretch may Vary considerably, dependcomprising iodine and having on one or bothsur- 'ing upon the purpose to which the sheet is to be faces an integrally formed protective layer comput, and generally speaking,a stretch oi' four or prising the product of esteriflcation of the mateilve times is quite satisfactory. It has also been rial of said sheet with a polybasic acid or a polyfound advantageous to follow the stretching opbasic acid derivative. eration with a baking treatment, as for example Afurther object is to provideanimproved lightby heating the sheet at approximately 145 C- polarizing sheet material of the above type wherefor 10 t0 15 minutes- It Should. however, be in said protective surface layer comprises a crossunderstood that these steps are Subject t0 Wide linked ester of polyvinyl alcohol, to provide such (Cl. iis-65) a sheet wherein said layer comprises a crosslinked polyvinyl alcohol ester of a polybasic acid or a derivative of such an acid and particularly an inorganic polybasic acid, and to provide such a sheet wherein said layer comprises a polyvinyl borate.

A still further object is to provide an improved light-polarizing material as outlined above wherein the material comprising said protective layers extends throughout the body of said sheet or film.

Additional objects and advantages will in part appear and in part be pointed out in the course of the following description oi one or more embodiments of the invention, which are given as nonlimiting examples, in connection with the accompanying drawing. which is a diagrammatic view illustrating an arrangement of apparatus suitable for use in the practice of the invention.

Patent No. 2,237,567 discloses the production of light-polarizing material in sheet form in a variety of ways, including the application of a solution of iodine or a dichroic dye to a sheet of polyvinyl alcohol and at some time during said process stretching said sheet to orient the mole- 2 cules therein. In accordance with the present invention, it has been discovered that light-polarizing material of greatly improved stability, not only against heat but also against moisture and ultra-violet radiation. may be produced by providing said sheets with an integrally formed surface layer comprising the product of esteriilcation of the material of said sheets with a polybasic acid, such for example as boric acid, or with a variation without in any way departing from the essentials of the invention.

vThe stretched and baked sheet may then be rolls Il through tank l5 containing a relatively strong iodine solution I 8. The composition of solution I6 may be varied considerably, but it should preferably have a relatively high ratio of iodide to iodine, and illustrative examples of satisfactory solutions will be given hereinafter. The time of treatment with solution i8 will depend both upon the composition of said solution and the desired density of stain to be obtained.

After sheet I0 has been stained, it should preferably be at least partially dried, as for example by passing through wiper elements i8 to remove the surplus iodine solution from the surfaces thereof. The next step of the process of the invention is to treat the sheet with the desired esterliying agent. This may conveniently be done 3 by passing sheet I! into a second tank 2l containing asolution 22 of said agent, and in accordancewith this embodiment of the invention particularly satisfactory results have been ob tained with a concentrated solution of boric acid. The time of treatment is not critical. It appears that the reaction is substantially complete in less than one minute, and in general from 30 to 45 seconds will be ample. Preferably this step will' be followed by a washing step for the purpose of removing the surplus acid solution from the surface of the sheet and preventing precipitation as it dries. Thus in the drawing, sheet III is shown as passing from tank 20 into a third tank 24 containing a washing solution 25, which may comprise either water or a suitable non-solvent of polyvinyl alcohol, such for example as acetone. The washing step should preferably be, relatively brief, i. e., of the order of five to ten seconds, since prolonged contact of the treated sheet with water may result in hydrolysis and dissolving of the boric vacid from sheet l0. After the washing step, the sheet should be dried in any desirable way, as for example by wiping, prolonged ex posure to the air, or moderate heating, before passing to take-up roll 28.

It appears that the result of the above outlined processis to convert a minute layer adjacent the surface of sheet l into the reaction product of polyvinyl alcohol and the reagent used in solution 22, and the properties of said product indicate that it is a cross-linked ester of polyvinyl alcohol. Thus in the example given above, the finished sheet I0 will be found to have an integral surface layer comprising a polyvinyl alcohol ester of boric acid, i. e., a polyvinyl berate, which is highly unreactive to heat and other forces affecting the stability of the iodine stain within the sheet. It is resistant to water, although on prolonged exposure thereto it will tend to hydrolyze, and it is substantially impervious to water vapor and also appears to seal the iodine in the body of the sheet in the same way that it seals out moisture. Said layer is so integral a part of the sheet that it is substantially' inseparable therefrom. Its thickness, particularly with relation to the total thickness of the sheet, depends upon the length of time the reaction is permitted to run and also on the initial thickness of the sheet. Since as noted the formation of the reaction product on the surface of the sheet tends to slow penetration of the body vof the sheet by the reagent, in a relatively thick sheet the esterifled layer tends to remain thin in relation to the total thickness of the sheet. In a `very thin sheet, however, the reaction may go to substantial completion to give, in the above example, a dichroic sheet of polyvinyl borate.

It is believed that the reaction by which the protective layer of the invention is formed in the 6 above example involves the condensation of each of the hydroxyl groups'in the boric acid with a hydroxyl group of the polyvinyl alcohol to form water, and that the boron atoms then serve to cross-link adjacent carbon chains of the polyvinyl alcohol through the remaining oxygen atoms. This reaction may be accompanied by some decrease in the degree of molecular orientation of the reacted layer. When, for example, the dichroic substance in the sheet comprises iodine, this in turn may cause a loss in the transmission of the sheet running as high as. from to 10% of incident light, and the predominant color of-the sheet also changes towards the blue.` This decrease in transmission may in some CaSeS 4 be desirable, particularly for the production of light-polarizing material of high polarizing ef-g; ciency but relatively low transmission. If de-'Q sired, however, this result may be prevented by modifying the above outlined process as described below.

As has already been mentioned, the above described esteritlcation process of the invention primarily affects the surface of sheet l0, and it appears that the borate or other cross-linked product formed during thefprooess tends to prevent further penetration of the sheet by the acid solution. The darkening result above mentioned may accordingly be prevented by eliminating the iodine from so much of the surface layers of the sheet as will be penetrated by the acid solution. One way of accomplishing this result is to wash sheet I0 with water after treatment in solution I5 but before treatment in solution 22. The time for this washing treatment depends primarily upon the hardness of the sheet and may vary, for ex- Iample, from l0 to 30 seconds. A simple way of determining the proper time is first to test portions of the sheet by washing for dierent periods followed by treatment in solution 22. When the latter step does not cause darkening or bluing of the sheet, it has been sumcientiy washed. Fol` lowing this washing step, the sheet may be dried' if desired, but this step i-s not necessary, and the sheet may be passed directly into solution 22.

Alternatively to the above step of washing the sheet with water, it may be washed instead with a stronger solvent for iodine, such for example as a solution of gelatin in water. The concentration of said solution may be relatively dilute, and satisfactory results have been obtained with approximately .5 gram of gelatin ln cc. of water. Other solvents for iodine may be used, such for example as, methanol, and the time of this washing treatment will depend to a considerable extent on the degree of solubility of iodine inthe wash,- ing solution used. For example, treatment withl methanol for as little as one second will generally be found satisfactory, and the sheet may then be passed directly into the acid solution either with or without preliminary drying. x

When the product of the invention is produced by any of the above washing steps, preferred results will be obtained if the initial staining of the sheet brings about a somewhat more dense color than is desired in the finished product, since said washing step causes removal of a part of the stain. The composition of solution I'B is in part deter- I mined by whether the washingstep will be with water or with a stronger solvent for iodine, al though in any case said 'solution should have a' relatively high proportion of iodide to iodine.l Examples of satisfactory ranges are as follows, although it is to be understood that they are given solely for illustrative purposes and not in any way as limitations of the invention. v

Iodine grams-- 5 Ammonium iodide do- 10-50 Water cc 50-200 It should be noted that the use of iodine solutions as strong as the above examples may tendto produce a yellowish tinge in the sheet treated' therewith, but this tendency may also be overcome by suitable control of the washing step of the invention. That is to say, if the washing step be stopped just prior to the removal 4of sumcient surface. stain to prevent any bluing by acid solu. tion 22, the slight bluish tinge produced by said acid S0lut10n will be found to neutralize the yel- SEARCH ROOM rCHa-CH-I- Lil.

and particularly with mixed polymers and other derivatives of polyvinyl alcohol such as the acetals and ketals. 'I'he term polyvinyl oxy compound as used herein and in the claims is to be understood as being limited to compounds containing the above atomic structure, and the term acetals and ketals of polyvinyl alcohol is to be understood as generic to the class of resins formed from polyvinyl acetate by the successive or combined steps of hydrolysis and condensation with aldehydes and ketones respectively. It should be pointed out, however, that when the invention is practiced with mixed polymers or derivatives of polyvinyl alcohol, only incomplete derivatives should be used, that is to say, some of the hydroxyl groups of the polyvinyl alcohol should remain unreacted in order to enter into the subsequent esterification reaction.

Boric acid is a preferred material for use in the practice oi the present invention, for a variety of reasons. Its reaction product with polyvinyl alcohol is particularly suitable for the purposes of the invention, for the reasons already discussed. The reaction goes very easily at room temperature using only water as solvent, and it is easily controlled to any desired degree of completion. Of particular importance is the fact that neither the reaction itself nor the conditions of reaction affect the polarizing properties oi' the sheet, except for the above mentioned darkening effect the ready control of which is also described. On the other hand, comparable results may also be obtained with derivatives such as boronhalides capable of esterifying an alcohol. In the latter case it will be apparent that the by-product of the vreaction will be a halogen acid instead of water.

Inv the production of the improved light-polarizing sheets of the present invention, the step of molecular orientation of the sheet, and preferably also the step of incorporating the iodine or other dichroic substance therein, should preferably be carried out before treatment with the acid solution. 'I'he surface coating of the products of the invention has little extensibility when dry and tends to crack and thus be rendered useless if the steps of the process are carried out in different order. On the other hand, the extensibility of the acid-treated sheet increases if the sheet is swelled with a suitable solvent or swelling agent, and a useful degree of molecular orientation can be obtained by stretching such sheets when swelled. Comparable results may be obtained by casting boric acid or a similar agent in a sheet of the desired plastic and then swelling and stretching the sheet. It will be apparent that this provides an alternative method for forming a substantially uniform sheet of the cross-linked material, and said sheet may be made light-polarizing by the incorporation of a suitable dichroic sucstance therein, for example by staining with CFI iodine as-explained above in connection with the drawing. These and similar modincations will doubtless be apparent to those skilled in the art and are to be construed as coming within the scope of the invention.

Since certain modifications in the product which embody the invention may be made withlout .departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A light-polarizing sheet comprising a molecularly oriented linear polyvinyl compound selected from the group consisting of polyvinyl alcohol, the polyvinyl acetals and the polyvinyl ketals; said sheet having an oriented dichroic sorption com plex of said polyvinyl compound and iodine incorporated therein, and a complex of boric acid and said polyvinyl compound .present on at least one surface of the sheet in a predetermined concentration and extending from said surface into the sheet in a progressively attenuated concentration, said boric acid-polyvinyl compound complex forming a layer which is heat and moisture res-istant, whereby the said iodine-polyvinyl compound complex is protected from heat and moisture.

2. A light-polarizing sheet comprising a molecularly oriented linear polyvinyl alcohol. said sheet having an oriented dichroic polyvinyl alcohcl-lodlne-sorption complex incorporated therein, and a poylvinyl alcohol-boric acid complex present on at least one surface of the sheet in a predetermined concentration and extending from said surface into the sheet in a progressively attenuated concentration and forming a layer which is heat and moisture resistant, whereby the said polyvinyl alcohol-iodine sorption complex is protected from heat and moisture.

3. A light-.polarizing sheet comprising a molecularly oriented linear polyvinyl butyral, said sheet having an oriented dichroic polyvinyl butyraliodine-sorption complex incorporated therein, and a poylvinyl butyral-boric acid complex present on at least one surface of the sheet in a predetermined concentration and extending from said surface into the sheet in a progressively attenuated concentration and forming a layer which is heat and moisture resistant, whereby the said polyvinyl butyral-iodine sorption complex is protected from heat and moisture.

4. A light-polarizing sheet comprising a molecularly oriented linear polyvinyl compound selected from the group consisting of polywinyl alcohol, the polyvinyl acetals and the polyvinyl ketals, said sheet having an oriented dichroic sorption complex of iodine and said polyvinyl compound incorporated therein and a complex of boric acid and said polyvinyl compound present on each surface of the sheet in a predetermined concentration, and forming layers which extend from each said surface into the sheet in progressively attenuated concentration and which are heat and moisture resistant, whereby the said iodine-polyvinyl compound complex is protected from heat yand moisture.

5. A thin light-polarizing sheet comprising a molecularly oriented linear polyvinyl compound selected from the group consisting of polyvinyl alcohol, the polyvinyl acetals and the vpolyvinyl ketals, said sheetV having an oriented dichroic sorption complex of iodine and said polyvinyl compound incorporated therein, said sheet also Mueve having incorponted therein end adjacent eachy surface thereof in predetermined concentration e complex of seid polyvinyl compound and boric acid which is heet and moisture resistant. said polyvinyl compound-boric acid complex forming a layer adjacent each surface ot seid sheet and extending from each said surface into seid sheet, whereby the said iodine-polyvinyl compound complex is protected from heat and moisture.

MARK HYMAN, Jn. CUTLER D. WEST.

REFERENCES CITED The following references are of record in the me of this patent:

8 UNn'mn snm m'rnrrs Numben Name Date Hermien et al Mar. 2, 1937*: Herman et al. Aug, 2, 1938" Land Apr. 8, 19411 Rogers Sept. 16, 1941 Strain Oct. 27, 1942 Kirby Mar. 2, 1943- Muskat et al June 22, 1943 Irany Aug. 10, 1943 Dahle Aug. 24, 1943 Vierling et al Mar. 14, 1944 Dhle Oct. 17, 1944 Thomas May 15,1945 

